Process for the preparation of acrylic acid esters



ilite 3,065,260 PROCESS FOR THE PREPARATION OF ACRYLIQ ACID ESTERS Wiiheim C. Kuhn and Karl Zeiie, Ingelheim (Rhine), Germany, assignors to C. H. Boehringei' Sohn, Ingelheim (Rhine), (Germany, a limited partnership of Germany No Drawing. Filed Aug. 15, I961, Ser. No. 131,499 4- Claims. (Cl. 269-436) This invention relates to a novel process for the preparation of acrylic acid esters, and especially of acrylic acid lower alkyl esters, by rapid thermal decomposition of lactic acid ester Xanthates.

A number of different methods for the preparation of acrylic acid esters from lactic acid derivatives have been described in prior patents and in the literature. For instance, it is known that acrylic acid esters can be prepared by dehydration of lactic acid esters and that the dehydration reaction is facilitated by previously esterifying the lactic acid ester at the hydroxyl group. Acrylic acid esters have also been prepared by reacting lactic acid with sodium chlorosulfonates and thereafter subjecting the reaction products to thermal cleavage.

However, these prior art methods of preparing acrylic acid esters from lactic acid or its derivatives, while being satisfactory for laboratory use, are burdened w th considerable disadvantages and limitations when applied to large scale commercial production and are therefore uneconomical; for example, they produce very poor yields of acrylic acid esters and require the use of very high reaction temperatures as Well as costly reaction assistants.

It is an object of the present invention to provide a method of preparing acrylic acid esters from lactic acid ester derivatives, which is economically feasible on an industrial scale.

It i another object of the present invention to provide a simple process whereby very high, virtually quantitative yields of acrylic acid esters can be obtained from lactic acid ester derivatives without the necessity of using excessively high reaction temperatures or costly reaction assistants.

Still another object of our invention is to provide a highly economical industrial process which produces not only high yields of acrylic acid esters but also high yields of other industrially valuable substances as side products.

Other objects and advantages of the invention will become readily apparent as the description thereof proceeds.

We have discovered that acrylic acid esters, along with carbonyl sulfide and thio-compounds, such as alkyl mercaptans and thioalkyl-carboxylic acid esters, are obtained in a surprisingly easy manner by rapidly, virtually instan taneously, heating small quantities of a lactic acid ester xanthate of the formula CHa-CHOOOR wherein R is a hydrocarbon radical, preferably lower alkyl, and R is selected from the group consisting of alkyl and carb-alkoXyalkyl, to a temperature of 180240 C. and condensing the gaseous decomposition products released thereby.

The rapid heating of the starting material to the above indicated temperature range may be accomplished either by contacting the lactic acid ester Xanthate dropwise with a solid surface which is at a temperature of ISO-240 C., or by introducing the lactic acid ester xanthate dropwise into an inert liquid medium heated to ISO-240 C., said liquid medium having a boiling point substantially above the temperature to which the lactic acid ester Xanthate is to be heated. The rapid heating effect of the solid surface method may be further enhanced by spreading a thin layer of finely ground or powdered glass or the like over the surface.

In other words, the decomposition of the lactic acid ester Xanthate under the conditions described above can be represented by the following illustrative reaction equation:

wherein R and R have the meanings defined above.

This reaction has a certain structural resemblance to the so-called Chugaev reaction [Berichte 32, 3332 (1899); the Merck Index, 7th ed., page 1414] which involves the formation of olefins from alcohols, without rearrangement, through the decomposition of methyl xanthates prepared from the alcohols:

However, this is where the similarity between the two reactions ends. The Chugaev method is inefiicient in that it produces very poor yield of olefins, and it is therefore useful substantially only as a laboratory method for qualitative analysis of organic unknowns.

The reaction according to the present invention is also structurally similar to the thermal decomposition reaction of alkyl-a-acetoxypropionate according to C. H. Fisher and E. M. Filachione, described in US. Department of Agriculture publication AIC-279 (1950), pages l32, which may be represented by the following reaction equation:

However, this reaction is also rather inefficient with respect to the yields of acrylic acid ester obtained therewith, and moreover, it requires extremely high operating temperatures.

In contrast thereto, the process of the present invention is surprisingly efficient in that it produces virtually quantitative yields of the desired products, and furthermore it proceeds at relatively low reaction temperatures.

Under the above described conditions the decomposition of the lactic acid ester Xanthates into acrylic acid esters, carbonyl sulfide and alkyl mercaptans or thioalkylcarboxylic acid esters not only proceeds very smoothly, but the yield of these decomposition products is virtually quantitative. In other words, our novel procedure produces not only virtually quantitative yields of the primary reaction product, i.e. the desired acrylic acid ester, but virtually quantitative yields of the side products which themselves are valuable industrial chemicals. All three of the thermal decomposition products are gaseous under the specified temperature conditions and may very readily be separated from each other.

As already indicated, the inert liquid medium into which the lactic acid ester Xanthate starting material is introduced dropwise in the liquid method of achieving rapid heating to the thermal decomposition temperature should have a boiling point substantially higher than the intended thermal decomposition temperature. While a number of readily available substances meet these requirements, the following are preferred examples: paraffin oil (liquid petroleum); silicone fluids, i.e. polysiloxanes with a boiling point of over 300 C., dibutylphthalate; oleic acid; oleic acid butyl ester; and mixtures of diphenyl and diphenyl oxide, such as that of the commercial product sold under the name of ""Diphyl.

The apparatus for performing the process according to the present invention should be provided with means for rapidly and promptly withdrawing the three gaseous decomposition products. In this manner, side reactions between the very reactive gaseous products can be con pletely avoided.

The three decomposition products may be recovered in the form of a liquid mixture by condensing the gaseous mixture after withdrawing it from the reaction vessel; or each decomposition product may be recovered separately by passing the product mixture through a distillation column connected to the reaction vessel. However, the separation may also be effected by any other suitable separation procedure based on the physical and chemical properties of the three products.

The lactic acid ester xanthates used as starting niateri-als in the process according to the present invention may be prepared by customary methods, such as by xanthation of lactic acid esters with carbon bisultide and an alkali metal hydroxide.

The following examples shall further illustrate the present invention with the aid of representative embodiments. It should be understood, however, that the invention is not limited solely to these illustrative embodimerits.

EXAMPLE I 194 parts by weight of carbomethoxyethyl-xanthic acid methyl ester of the formula were added dropwise over a period of one hour to a mixture consisting of 50 parts by weight of oleic acid butyl ester and 0.5 part by weight of hydroquinone which was maintained at 220 C. The rate of dropwise addition was such that a substantially uniform evolution of gaseous decomposition products was maintained. The gaseous reaction products were rapidly withdrawn and condensed. The condensed liquid mixture, amounting to 187 parts by weight, was then fractionally distilled in a column filled with Raschig rings at a temperature ranging from -50 to +85 C. The rate of dropwise addition of the xanthic acid ester was controlled in such a manner that the rate of evolution of decomposition products was substantially equal to the rate of distillation of the condensed liquid mixture. The following fractions were obtained:

56.4 parts by weight of carbonyl sulfide (COS), Bl. -48 C., corresponding to a yield of 94% of theory;

44 parts by weight of methyl mercaptan (Cl-1 8E), Bi. 7.6 C., corresponding to a yield of 93.5% of theory; and

77 parts by weight of acrylic acid methyl ester (CH =CH-COOCH B.P. 7882 C., corresponding to a yield of 92% of theory.

EXAMPLE II 111 parts by weight of carbeth-oxyethyl-xanthic acid ethyl ester of the formula which corresponds to a yield of 96% of theory. Fractional distillation of this liquid mixture yielded:

48 parts by weight of acrylic acid ethyl ester (CH :CHCOOC H B.P.=97-100 (3., corresponding to a yield of 96% of theory;

29 parts by weight of carbonyl sulfide, corresponding to a yield of 97% of theory; and

29 parts by weight of ethyl mercaptan (C H SH), corresponding to a yield of 94% of theory.

EXAMPLE III 100 parts by weight of oleic acid butyl ester containing 1 part by weight of hydroquinone were placed into a flask which was connected to a distillation column. The contents of the flask were heated to 220 C., and then 390 parts by weight of carbomethoxyethyl-xanthic acid methyl ester were added dropwise to the hot oleic acid butyl ester over a period of about one and one-half hours. The gaseous decomposition products were continuously withdrawn from the flask and passed through the distillation column. At the head of the column parts by weight of carbonyl sulfide were obtained, which corresponds to a yield of 92% of theory. From the lower section of the column 91 parts by weight of methyl mercaptan were withdrawn, which corresponds to a yield of 94% of theory. The distillation residue, withdrawn from the bottom of the column, was further distilled and yielded parts by weight of acrylic acid methyl ester, B.P. 78-82 C., which corresponds to a yield of 91% of theory.

EXAMPLE IV 40 parts by weight of a polysiloxane fluid having a boiling point above 300 C., which contained 0.5 parts by weight of hydroquinone, were heated to 220 C. and maintained at that temperature while 104 parts by weight of carbomethoxyethyl-xanthic acid ethyl ester were added dropwise thereto over a period of about two hours. The gaseous decomposition products were continuously withdrawn and condensed, yielding 97.2 parts by weight of a liquid mixture which was fractionally distilled. The following fractions were obtained:

39.5 parts by weight of acrylic acid methyl ester, corresponding to a yield of 92% of theory;

28 parts by weight of ethyl mercaptan, corresponding to a yield of 91% of theory; and

27.5 parts by weight of a carbonyl sulfide, corresponding to a yield of 92% of theory.

EXAMPLE V 118 parts by weight of carbisopropoxyethyl-xanthic acid ethyl ester of the formula i CHa-CH-C O O CH were added dropwise over a period of forty minutes to 50 parts by weight of oleic acid butyl ester which were maintained at 240 C. The gaseous decomposition products were continuously withdrawn and condensed, yielding 110.5 parts by weight of a liquid mixture, corresponding to a yield of 93% of theory. The mixture was fractionally distilled, whereby the following fractions were obtained:

26.2 parts by weight of carbonyl sulfide, corresponding to a yield of 87.5% of theory;

27.1 parts by weight of ethyl mercaptan, corresponding to a yield of 87.5% of theory; and

57.1 parts by weight of raw acrylic acid isopropyl ester,

corresponding to a yield of 100% of theory.

The raw acrylic acid ester was purified by again distilling it; the purified ester, having a boiling point of 107-l10 C., was obtained with a yield of 52 parts by weight, corresponding to a yield of 91.5% of theory.

EXAMPLE VI 28 parts by weight of carbethoxyethyl-xanthic acid carbethoxymethyl ester of the formula CHaOHOOOO H,-,

were added dropwise over a period of thirty minutes to a polysiloxane fluid having a boiling point above 300 C., which had been heated to 250 C. and was maintained at that temperature during the entire period of addition of the lactic acid ester xanthate. The gaseous decomposition products were continuously withdrawn and condensed into a liquid mixture. This mixture was then fractionally distilled, whereby the following fractions were obtained:

5.3 parts by weight of carbonyl sulfide, corresponding to a yield of 88.5% of theory;

9.3 parts by weight of acrylic acid ethyl ester, B.P. 98 100 0, corresponding to a yield of 93% of theory; and

8.5 parts by weight of thioglycollic acid ethyl ester (HSCH -COOC H 2 additional parts by weight of thioglycollic acid ethyl ester were recovered from the polysiloxane fluid, making the total yield of this compound 10.5 parts by weight, which corresponds to a yield of 84% of theory.

EXAMPLE VII 75 parts by weight of carbobutoXyethyl-Xanthic acid ethyl ester of the formula were added dropwise over a period of one hour to a mixture of 50 parts by Weight of oleic acid butyl ester and 0.55 parts by weight of hydroquinone at 240 C. The oleic acid ester mixture was maintained at that temperature throughout the period of addition of the lactic acid ester xanthate. The gaseous decomposition products were continuously withdrawn and condensed into a liquid mixture. Fractional distillation of this liquid mixture produced the following fractions:

17.3 parts by weight of carbonyl sulfide, corresponding to a yield of 96.5% of theory;

16.8 parts by weight of ethyl mercaptan, corresponding to a yield of 90% of theory; and

35 parts by weight of acrylic acid butyl ester B.P. 145-147 C., corresponding to a yield of 91% of theory.

While we have illustrated our invention with the aid of certain specific embodiments, it will be readily apparent to others skilled in the art that our invention is not limited to these embodiments and that various changes and modifications may be made without departing from the 6 spirit of the invention or the scope of the appended claims.

We claim: 1. The method of preparing acrylic acid esters of the formula CHFCH-COOR wherein R is a hydrocarbon radical, which comprises sub stantially instantaneously heating a lactic acid ester Xanthate of the formula wherein R has the meaning defined above and R is selected from the group consisting of alkyl and carbalkoxyalkyl, to a temperature of l240 C.

2. The method of preparing acrylic acid esters of the formula CH =CHCOOR wherein R is lower alkyl, which comprises contacting small amounts of a lactic acid ester xanthate of the formula CH3CHOOOR o s n' wherein R has the meaning defined above and R is selected from the group consisting of lower alkyl and carblower alkoxy-lower alkyl, with a solid surface which is heated to a temperature of 240 C.

4. The method of preparing acrylic acid esters of the formula CH =CHCOOR wherein R is lower alkyl, which comprises immersing small amounts of a lactic acid ester Xanthate of the formula OH3-CHCOOR wherein R has the meaning defined above and R is selected from the group consisting of lower alkyl and carblower alkoXy-lower alkyl, into a hot inert liquid which is heated to a temperature of ISO-240 C., said inert liquid having a boiling point substantially higher than the temperature to which it is heated.

No references cited. 

1. THE METHOD OF PREPARING ACRYLIC ACID ESTERS OF THE FORMULA 